1. Field of the Invention
This invention relates generally to the field of methods and apparatus for synchronizing signals. More particularly the present invention relates to the field of methods and apparatus for synchronizing two or more pulsed lasers so that their respective pulses are synchronized in time to within less than 1 picosecond.
2. The Prior Art
Mode-locked laser systems, when combined with optical pulse compression techniques, are capable of generating extremely short optical pulses, making such devices attractive for a variety of time-resolved measurements such as time-resolved spectroscopy, electro-optic sampling, photo-enhanced electron-beam probing of integrated circuits, and electromagnetic transient measurements in semiconductors. Such measurements are often made by pumping or exciting the target with the pulsed laser and subsequently probing the target with a delayed portion of the same optical pulse, suppressing the effect of pulse timing fluctuations.
Under certain circumstances it is most advantageous to carry out such pump-probe measurements using different wavelengths (and different light sources) for the pump beam and the probe beam. In such systems, fluctuations of the laser pulse timing degrade the time resolution in proportion to the uncertainty of the pulse arrival time. Accordingly, the use of two (or more) lasers has not, until recently, provided an attractive solution for dual-wavelength pump-probe measurements because the pulse sequence from one laser is, in general, not well correlated with that from the other because of the presence of timing jitter in the outputs.
Passively mode-locked lasers have often been preferred when the shortest possible pulse durations are required. The random nature of the pulse evolution in a passively mode-locked system further complicates the problem of synchronizing two such systems. It is well known that the pulse repetition rate of a passively mode-locked laser is determined by its cavity length. This means that any random fluctuation in the effective cavity length will result in a corresponding variation in the pulse repetition frequency, which manifests itself as phase noise (also referred to as "pulse timing jitter" or simply "jitter") on the mode-locked output. The rms timing jitter .DELTA.t.sub.rms that arises from a cavity length change of .DELTA.1.sub.pk--pk in a cavity of length L, at a frequency F, is given by the equation: ##EQU1## Accordingly, for a cavity of length L=1.875 meters, which corresponds to a nominal 80 MHz repetition rate, a change in cavity length of only 10 nm (10 nm pk-pk jitter) at a 200 Hz fluctuation rate will result in an rms pulse timing jitter of about 1.5 pS (picoseconds). Conversely, to reduce the pulse timing jitter at 200 Hz to the order of the length of a typical pulse duration or less (.ltoreq.100 fS (femtoseconds)), the cavity length must be controlled to an accuracy of approximately 0.7 nm.
In order to obtain a cross-correlation between the output of two passively mode-locked lasers it is desirable that their cavity periods be precisely matched. Furthermore, the pulses must be synchronized at some initial time. If, at some later time, one of the laser cavity periods changes then its pulses will walk off temporarily due to the change in repetition frequency. Simply matching the two cavity lengths again is not sufficient to re-establish the cross-correlation because although both lasers may have the same repetition frequencies, the pulse trains can be out of synchronism. The cavity length must therefore be adjusted to bring the pulses back into phase and then maintain their temporal coincidence.
Prior art schemes for jitter reduction have not been successful in reducing jitter substantially below about 1 pS. See, e.g., "Reduction of timing fluctuations in a mode-locked Nd:YAG laser by electronic feedback", Rodwell et al., Optics Letters, Vol. 11, No. 10, p. 638 (October, 1986); "Timing-Jitter Stabilization of a colliding-pulse mode-locked laser by active control of the cavity length", Darack et al., Optics Letters, Vol. 16, No. 21, p. 1677 (Nov. 1, 1991); "Series 1000 Timing Stabilizer", Lightwave Electronics. Accordingly, a need exists for an improved method and apparatus for reducing timing jitter well below 1 pS.